xmalloc.c: introduce xgrowarray helper function

In order to simplify dynamic array management code.

* xmalloc.h (xgrowarray): New declaration.
* xmalloc.c (xgrowarray): New function.

xmalloc.c

@@ -89,6 +89,28 @@ xreallocarray(void *ptr, size_t nmemb, size_t size)
 	return p;
 }
 
+void *
+xgrowarray(void *const ptr, size_t *const nmemb, const size_t memb_size)
+{
+	/* this is the same value as glibc DEFAULT_MXFAST */
+	enum { DEFAULT_ALLOC_SIZE = 64 * SIZEOF_LONG / 4 };
+
+	size_t grow_memb;
+
+	if (ptr == NULL)
+		grow_memb = *nmemb ? 0 :
+			(DEFAULT_ALLOC_SIZE + memb_size - 1) / memb_size;
+	else
+		grow_memb = (*nmemb >> 1) + 1;
+
+	if ((*nmemb + grow_memb) < *nmemb)
+		die_out_of_memory();
+
+	*nmemb += grow_memb;
+
+	return xreallocarray(ptr, *nmemb, memb_size);
+}
+
 char *
 xstrdup(const char *str)
 {

xmalloc.h

@@ -42,6 +42,23 @@ void *xmalloc(size_t size) ATTRIBUTE_MALLOC ATTRIBUTE_ALLOC_SIZE((1));
 void *xreallocarray(void *ptr, size_t nmemb, size_t size)
 	ATTRIBUTE_ALLOC_SIZE((2, 3));
 
+/**
+ * Utility function for the simplification of managing various dynamic arrays.
+ * Knows better how to resize arrays. Dies if there's no enough memory.
+ *
+ * @param[in]      ptr       Pointer to the array to be resized. If ptr is NULL,
+ *                           new array is allocated.
+ * @param[in, out] nmemb     Pointer to the current member count. If ptr is
+ *                           NULL, it specifies number of members in the newly
+ *                           created array. If ptr is NULL and nmemb is 0,
+ *                           number of members in the new array is decided by
+ *                           the function. Member count is updated by the
+ *                           function to the new value.
+ * @param[in]      memb_size Size of array member in bytes.
+ * @return                   Pointer to the (re)allocated array.
+ */
+void *xgrowarray(void *ptr, size_t *nmemb, size_t memb_size);
+
 /*
  * Note that the following two functions return NULL when NULL is specified
  * and not when allocation is failed, since, as the "x" prefix implies,